The invention relates to an apparatus for spreading open cross-bottoms on tubular members which are to be further processed to form cross-bottom sacks or bags, said apparatus having two spreader members which rotate synchronously and symmetrically relative to the plane of the tube ends to be formed and which are driven by a nonuniformity drive transmission and which engage into and spread open the tube ends.
An apparatus of this kind is known from the applicant's German Pat. No. 2,062,944 in which crescent-shaped spreader members which rotate about shafts which are made pivotal by means of a universal joint, even during operation, can be driven by way of a non-uniformity drive transmission. In this apparatus the pivotal movement of the spreader members about axes extending parallel to the direction of conveyance of the tube portions, in the direction of increasing the angle of inclination of the spreader members, serves to improve the spreading operation by adaptation of the spreading movement which is on an elliptical path in projection on the bag or sack bottom to be opened. The ellipse of the path is distorted by the pivotal movement, into an approximation to the ideal spreading movement. In the known apparatus provision is also made for pivotability of the axis of the spreader members so that they are pivotal on a subatantially conical path about the straight line extending through the two pivot points, in the direction of displacing the point of closest approach of the spreader members on a circular track which lies in the plane of conveyance of the ends of the tube portions. By means of this pivotability, the path of movement of the spreader members can be varied to provide for the best possible entry into the ends of the tube portions, and good forming of the bottom folds. The curved paths of the spreader members are to be so adjusted or to be varied by pivotal movement that the spreader members engage into the tube portion as it passes by, with as great as possible an approach of the crescent members, and stretch the corner tucks out as well as possible, by approaching the fold edges thereof. When the corners of large bottom squares are to be extended, it is at any event advantageous to pivot the shafts of the spreader members during the rotation thereof, to provide for a good stretching movement. While the above-described modes of pivotal movement of the shafts of the spreader members provide for optimum adaptation of their curved paths to the spreading operation, the non-uniform drive for the shafts of the spreader members, which is due to the universal joints, can be utilised for increasingly reducing their peripheral speed as they pass into the tube opening and for increasing their peripheral speed as they come out of the tube opening again, so that the peripheral speed component of the spreader members in the direction of conveyance of the tube portions substantially co-incides with their speed of conveyance. It is necessary for the speed of the rotating spreader members to be closely adapted to the speed of the tube portions as they pass, in order to ensure that the corner tucks are satisfactorily smoothed out and in order to prevent the spreading members from becoming hooked up, for example when forming the rear corner tuck. The universal joints which are used in the known apparatuses must be connected to the shafts of the spreader members at a rotary angle such that the non-uniformity in the drive to the spreader members, which is caused by the universal joints, is utilised to comply with the conditions required. Only if the non-uniform drive movement as predetermined by the universal joint is not sufficient, is a non-uniformity drive transmission provided in the known apparatuses. The rotary angular position of the non-umiformity drive transmission relative to the universal joints is so selected that its non-uniformity movement is added to or subtracted from that of the universal joints, as required, in order to achieve with a good degree of approximation the non-uniform peripheral speed required for synchronising the spreader members with the tube portion and for producing the spreading movement thereof. In the known apparatus adapting the non-uniform peripheral speed of the spreader members to the throughpassage speed of the tube members involves substantial difficulties as the degree of non-uniformity of the speed of movement and the phase position of the non-uniformity are predetermined, due to the structure involved, because of the use of universal joints. In order to provide for easy handling and multi-purpose use of the apparatus however, it is desirable to be able to adjust the spreader members easily to different bottom configurations.